CallbackArtistAnalyser.java

package com.insightfullogic.java8.answers.chapter9;

import com.insightfullogic.java8.examples.chapter1.Artist;

import java.util.function.Consumer;
import java.util.function.Function;

public class CallbackArtistAnalyser implements ArtistAnalyzer {

    private final Function<String, Artist> artistLookupService;

    public CallbackArtistAnalyser(Function<String, Artist> artistLookupService) {
        this.artistLookupService = artistLookupService;
    }

    public void isLargerGroup(String artistName, String otherArtistName, Consumer<Boolean> handler) {
        boolean isLarger = getNumberOfMembers(artistName) > getNumberOfMembers(otherArtistName);
        handler.accept(isLarger);
    }

    private long getNumberOfMembers(String artistName) {
        return artistLookupService.apply(artistName)
                                  .getMembers()
                                  .count();
    }

}